Noble gas

A chemical element in Group 18 of the periodic table, typically characterized by a filled outer electron shell and low chemical reactivity.

Periodic table: A tabular arrangement of chemical elements ordered by atomic number, with recurring properties organized into groups and periods. Group 18 places these elements together because their properties recur with similar outer-shell structures.
Ionization energy: The energy required to remove an electron from an atom or ion in the gas phase. Noble gases generally resist losing electrons because their occupied shells are especially stable.
Helium cryogenics: The use of liquid or gaseous helium to achieve and maintain very low temperatures. Helium’s exceptionally low boiling point enables cooling for superconducting magnets and other systems.
Halogen: A chemical element in Group 17, generally reactive and one electron short of a filled outer shell. Halogens sit beside noble gases and commonly gain the electron their neighbors already possess.
Valence electron: An electron in an atom’s outermost shell that participates in chemical bonding and reactions. A filled set of valence electrons underlies the group’s characteristic low reactivity.
Chemical inertness: A low tendency of a substance to undergo chemical reactions under specified conditions. This broader property describes the group’s reputation while allowing important exceptions.
Neon lighting: Lighting that produces colored electrical-discharge light in a sealed tube containing neon or another gas. Neon’s bright red-orange glow made it a distinctive source for signs.
Alkali metal: A chemical element in Group 1, typically reactive and characterized by one valence electron. Alkali metals readily lose an outer electron, opposite the noble gases’ stable configurations.
Electron shell: A region around an atomic nucleus in which electrons with similar energies are grouped. The outer shell’s occupancy distinguishes noble gases from neighboring elements.
Noble gas compound: A chemical compound containing at least one noble-gas atom bonded to another element. Such compounds show that filled shells do not make chemical bonding impossible.

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